Because it is perpetuating the myth that prayer is acceptable and valid. Every study done proves that prayer is ineffectual - in hospitals, people die or get well at the same rate whether or not prayer is used.
Prayer doesn't help you win the lottery or help your team win the "game".
Prayer is supposed to be done in private according to the Christian bible.
Prayer doesn't put out fires or fix disasters.
Prayer doesn't regrow severed limbs.
Prayer doesn't bring peace.
Prayer doesn't feed the starving children in Africa (or elsewhere).
Prayer doesn't make you a better person.
Prayer doesn't convert Satanists or cast out demons.
Prayer only comforts the person that is doing the praying, it doesn't comfort the person being "prayed for", which makes a prayer a bit selfish.
Prayer is a personal thing, not a group thing.
Teaching kids to pray is the same as teaching them to wish for things instead of going out and working for them.
Prayer is simply awkward.